1. Field of the Invention
This invention relates to a method of rolling steel rods and wires with grooveless rolls, including entry guides for holding a blank material in the gap between the rolls.
2. Description of the Prior Art
The term "steel rods and wires" used herein is intended to designate elongated square, rectangular or circular cross-sectional rods and wires of steel and non-ferrous materials.
The term "grooveless roll" or "caliberless roll" used herein means a roll which is not formed with a caliber or calibers in its barrel.
In rolling blank materials having square cross-sections to produce steel rods, wires or the like having rectangular or circular cross-sections, pairs of caliber rolls have hitherto been exclusively used. FIG. 1 of the accompanying drawings, illustrates a typical pass schedule, wherein, a blank material w having a square cross-section is rolled through square and parallelogram calibers s and d one or more times and thereafter rolled through calibers having substantially the same sectional shapes as the above to obtain square cross-sectional steel products p or alternately through oval and round calibers o and r to obtain circular cross-sectional steel products p'. In these cases, the material is generally subjected to continuous rolling wherein it passes through the rolls of a continuous series of rolling mills M.sub.1, M.sub.2, M.sub.3, . . . M.sub.n as shown in FIG. 2 (of the drawings).
When carrying out rolling operations with a continuous series of rolling mills including pairs of caliber rolls, the number of roll stands required for the reduction of the material is determined by dimensions of ultimate products and cross-sections of the blank materials. In the case where circular cross-sectional rods having an outer diameter of 20 mm are to be produced from blank materials of a square section having sides of 145 mm, for example, there are required six roughing, intermediate and finishing roll stands, respectively, which include pairs of rolls having calibers as shown in FIG. 1. Such rolling operations with caliber rolls will involve the following problems.
(1) When the rolls of a pair of caliber rolls are shifted from their alignment positions or the caliber roll centers and the centers of guide means for introducing materials to be rolled into the caliber rolls are shifted with respect to each other, protrusions in the form of fins in the longitudinal direction will occur on the materials delivered from the caliber rolls. These fins will collapse during the next rolling operations to cause defects such as overlaps on the surfaces of the rods.
(2) In order to avoid the above defects, it is necessary to set the rolls and guide means with high accuracy and this requires a long down time.
(3) The accuracy of the dimension and the shape of the caliber rolls directly affects the quality of the product to a great extent so that highly complex and highly expensive roll lathes are required to machine the caliber rolls.
(4) Differences in circumferential speed between the respective rolls of the pairs of caliber rolls give rise to frictional irregular wear, so that the rolls must be machined many times to correct the calibers with resulting increased cost.
(5) If the size of rods to be rolled is changed (for example, from a 16 mm outer diameter of circular cross-sectional rods to a 40 mm outer diameter rod, many caliber rolls must be changed and this increases the down time of the rolling mills. It is impossible to use a pair of caliber rolls over a wide range of sizes of rods to be rolled.
(6) If the gap between a pair of caliber rolls is unintentionally made smaller than a predetermined value, protrusions occur on the surfaces of the rolled materials, which collapse during the next rolling operations to form defects such as overlaps on the surfaces.
In order to avoid the above disadvantages of caliber rolling, a rolling method using caliberless rolls has recently been proposed wherein the blank materials are rolled by caliberless rolls mainly for the purpose of reducing the cross-sectional areas of the rods and these are then further rolled by caliber rolls for obtaining the ultimate shape of the products. FIG. 3 of the drawings, illustrates a basic pass schedule for this method, in which the caliberless rolls are used in upstream passes u and intermediate passes m immediately before the forming passes f and caliber rolls are used in the forming passes f.
When caliberless rolls are substituted for caliber rolls, the machining of the calibers is naturally not required and the damage and wear on the surfaces of the caliberless rolls are less than in the case of caliber rolls. Thus the life of the rolls is longer resulting in lower cost. Also, shorter down time is required, because a change of rolls is not required even if the shapes and sizes of the products to be rolled are changed. However, the caliberless rolls have the following disadvantages.
(1) Caliberless rolls do not restrain the materials in the width direction thereof because they do not have calibers. Thus elongation of the materials in the rolling direction is less than in the case of caliber rolls. In order to obtain elongation of the materials in the rolling direction substantially equivalent to that obtained with caliber rolls, the reductions must be increased. The increased reduction however, makes the flat ratio larger, which is defined as B.sub.o /H.sub.o shown in FIG. 4 of the drawings. Because of the excess flat ratio, the cross-section of the material is incorrectly deformed in the next caliberless roll pass as shown in FIG. 4, so that the overturn a/H increases depending upon the flat ratio B.sub.o /H.sub.o, which makes it impossible to continue the rolling operation.
(2) When the reduction is comparatively large, the free surfaces of the materials which are not in contact with the rolls bulge as shown in FIG. 5 of the drawings. If the bulge ratio which is defined as b/H.sub.o is too large, the overturn a/H becomes large which makes it impossible to effect the next rolling.
(3) When an existing rolling installation is changed from caliber rolling to caliberless rolling, the number of passes must be increased because of the reduced elongation of the material in the rolling direction. This decreases the productivity of the installation and in turn leads to an increase in the number of roll stands in continuous rolling mills.